Ordered phases in Ln-S systems (Ln = Tm, Yb)

Abstract

The structures of known defect ordered phases present in Ln-X systems (50-60 at.% X) are analyzed and systematized according to the concentration of cation vacancies as cubic and trigonal phases. The structures of the defect phases present in Ln-X systems (X = S, Se, Te) are modeled by ordering the cation vacancies along (111) in the basic NaCl structure (ao~5.6 A). The following structures were derived: α-NaFeO2 and defect structures on its basis ScI.37[]0.62S2, CaHo2Se4 (ahex = 2a0√2/4, Chex = 2a0√3 aR = 2a0√6/4, αR = 33ℴ), CaY2Se4 ( ahex = 2a0√2/2, chex = 2a0√3: aR = 2a0√2/2, αR = 60ℴ), and EuEr2Te4 ( ahex = a0√6/2, chex = a0√3 ). All these phases except the last one may be obtained from NaCl by using direct rhombohedral distortion, and the last phase is a superstructure of a rhombohedral cell with parameters aR ~ 2a0√2/2, αR ~ 60ℴ. The solutionmelt method (eutectic mixture 1.86NaCl-NiCl2 with Tm(Yb):S from 1:3 to 1:10) was used to synthesize poly crystalline samples containing two new trigonal defect phases of possible composition Ln1+x+[]1-xS2 with cell parameters a = 3.835(7), c = 18.89(4) A and a = 3.83(3), c= 18.78(1) A (I) and also a = 7.749(7), c = 18.46(3) A and a = 7.74(1), c = 18.41(3) A (II) in Tm-S and Yb-S systems, respectively. It is maintained that the crystal structure of phase I with x ~ 0.37 (space group R3m) is derived from the NaCl structure and is close to Sc1.37[]0.63S2 and the crystal structure of phase II with x~0.34 (space group P3cl) is derived from the Ni As structure. Possible ordering of cations with different formal charges accompanied by variation of symmetry is discussed.